Hydraulic system for an automatic gearbox of a motor vehicle

ABSTRACT

A hydraulic system for an automatic transmission of a motor vehicle. A high-pressure circuit, in which a pressure accumulator, at least one clutch, and gear selectors and a hydraulic pump, which can be operated by an electronic control unit and by which the accumulator pressure in the high-pressure circuit can be increased in charging operation. A clutch valve that can be operated by the control unit is arranged in a clutch path between the pressure accumulator and a clutch hydraulic cylinder of the clutch, using which clutch valve a hydraulic pressure applied to the clutch hydraulic cylinder can be adjusted, and a safety valve that can be operated by the control unit is arranged upstream of the clutch valve.

FIELD

The invention relates to a hydraulic system for an automatictransmission, more particularly a dual-clutch transmission, of a motorvehicle.

BACKGROUND

In a dual-clutch transmission, two sub-transmissions enable fullyautomatic gear changes without any interruption of tractive power.Torque is transmitted via one of two clutches, which connects the twosub-transmissions to the drive. The clutches and the gear selectors forengaging the gears are actuated via hydraulic cylinders, which arecontrolled hydraulically via a hydraulic system.

From DE 10 2014 003 083 A1 a generic hydraulic system is known, whichhas a pressure accumulator for supplying an accumulator pressure in thehydraulic system. In a clutch path leading from the pressure accumulatorto the clutch hydraulic cylinder, a clutch valve is positioned, whichcan be controlled by an electronic control unit and can be used toadjust the hydraulic pressure applied to the clutch hydraulic cylinder.The control unit is preferably assigned a pressure sensor (DE 10 2013003894 A1), with which the hydraulic pressure applied to the clutchhydraulic cylinder can be detected. The hydraulic system also includes ahydraulic charge pump, which delivers hydraulic fluid into the hydraulicsystem in a charging operation in order to increase the accumulatorpressure.

As mentioned above, a clutch valve, which can be operated by anelectronic control unit and can be used to adjust the hydraulic pressureapplied to the clutch hydraulic cylinder, is arranged in the clutchpath. The clutch valve can be adjusted between a closed position and aflow-through position. In the event of a malfunction of the clutchvalve, the risk exists that the clutch valve can no longer be moved intoits closed position, so that a high hydraulic pressure is continuouslyapplied to the clutch hydraulic cylinder. An additional safety valvethat can be operated by the control unit is to be provided for such acase of fault. The safety valve can be provided upstream of the clutchvalve and can decouple the clutch path from the pressure accumulatorwith respect to pressure in its closed position. In the flow-throughposition of the safety valve, the accumulator pressure can be applied tothe clutch path.

SUMMARY

The object of the invention is to provide a hydraulic system in whichthe operational reliability of the pressure accumulator can be ensuredwith reduced sensor system complexity.

The control unit includes a diagnostic module, using which a safetyvalve diagnosis can be carried out to ensure correct function of thesafety valve. During the safety valve diagnosis, the safety valve isswitched at a diagnosis start time from its flow-through position intothe closed position, whereby an actual pressure decrease resultsupstream of the safety valve. The diagnostic module includes an analysisunit, which compares the actual pressure decrease to a target pressuredecrease and recognizes a fault if a significant deviation is present.Such a fault can result, for example, because of soil deposits, becauseof which the safety valve can no longer be adjusted.

A corresponding pressure sensor can be associated with the electroniccontrol unit for detecting the actual pressure decrease. The pressuresensor for detecting a hydraulic pressure applied to the clutchhydraulic cylinder can preferably be used for this purpose. In normaldriving operation, the pressure sensor fulfills a safety function, inwhich it monitors whether the clutch is depressurized or pressurized.During the safety valve diagnosis, in contrast, the pressure sensorcarries out the detection of the above-mentioned actual pressuredecrease downstream of the safety valve in a double function. Thepressure sensor can preferably be arranged between the clutch valve andthe clutch hydraulic cylinder. For correct detection of the actualpressure decrease, it is preferable if the clutch valve is adjusted intoits through-flow position with a time delay before the above-mentioneddiagnosis start time. Moreover, it is preferable with regard to correctdetection of the actual pressure decrease if the hydraulic pump is incharging operation during the safety valve diagnosis, i.e., it isoperated at a speed to ensure a sufficiently high accumulator pressurein the high-pressure circuit. Moreover, it is preferable if the gearselector hydraulic cylinder is decoupled from the high-pressure circuitduring the safety valve diagnosis.

The actual pressure in the high-pressure circuit is preferably modulatedbetween an upper and a lower pressure threshold value during the normaloperation and preferably also during the safety valve diagnosis. Forreasons of cost, the maximum measurement range which can be detected bythe pressure sensor can be outside, i.e., below the actual accumulatorpressure. In this case, the actual accumulator pressure applied in thehydraulic cylinder is not read out from the pressure sensor during theabove-mentioned time delay, but rather an upper limiting pressure of themeasurement range. In the above configuration, the actual pressuredecrease that can be detected by the pressure sensor, at which theclutch pressure decreases to the ambient pressure, is thus identical tothe maximum pressure sensor measurement range.

The advantageous embodiments and/or refinements of the inventiondescribed above and/or reflected in the dependent claims may be usedindividually or in any desired combination with one another except, forexample, in the case of clear dependencies or incompatible alternatives.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention and its advantageous embodiments andrefinements along with the advantages thereof will be described ingreater detail with reference to the drawings.

In the drawings:

FIG. 1 is a block diagram of a dual-clutch transmission for a motorvehicle having seven forward gears and one reverse gear;

FIG. 2a shows a block diagram of a hydraulic system of a dual-clutchtransmission and a rough schematic of the structure of a pressureaccumulator;

FIG. 2b shows another block diagram of a hydraulic system of adual-clutch transmission and a rough schematic of the structure of apressure accumulator;

FIG. 3 is a block diagram showing the program blocks required forpressure accumulator and clutch path diagnosis in a diagnostic module;and

FIG. 4 contains graphs illustrating the pressure accumulator and clutchpath diagnosis;

FIG. 5 is a block diagram showing the program blocks required for gearselector path diagnosis in the diagnostic module;

FIG. 6 contains graphs illustrating the gear selector path diagnosis;

FIG. 7 is a block diagram showing the program blocks required foraccumulator volume diagnosis in the diagnostic module;

FIG. 8 contains graphs illustrating the accumulator volume diagnosis;

FIG. 9 is a block diagram showing the program blocks required forswitchover timing diagnosis in the diagnostic module;

FIG. 10 is a block diagram showing the program blocks required for valvespread diagnosis in the diagnostic module;

FIG. 11 contains graphs illustrating the profiles over time duringswitchover timing diagnosis and during valve spread diagnosis;

FIG. 12 is a block diagram showing the program blocks required forsafety valve diagnosis in the diagnostic module;

FIG. 13 contains graphs illustrating the profiles over time of relevantparameters during safety valve diagnosis;

FIG. 14 is a block diagram showing the program blocks required fordelivery volume flow diagnosis in the diagnostic module;

FIG. 15 contains graphs illustrating the profiles over time duringdelivery volume flow diagnosis; and

FIG. 16 shows an analysis unit into which the fault signals generated inthe fault memories can be read.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram of a dual-clutch transmission for a motorvehicle with all-wheel drive. This dual-clutch transmission has sevenforward gears (see circled numerals 1 through 7) and one reverse gearRW. In the following, the dual-clutch transmission will be describedonly insofar as is necessary for an understanding of the invention. Thedual-clutch transmission has two input shafts 12, 14, which are arrangedcoaxially to one another and can be connected to the drive source, forexample an internal combustion engine, alternatingly, via twohydraulically actuated multi-plate clutches K1, K2. Input shaft 14 isembodied as a hollow shaft, in which input shaft 12, embodied as a solidshaft, is guided. The two input shafts 12, 14 drive an axially paralleloutput shaft 16 and an intermediate shaft 18 embodied as a hollow shaftvia gear sets of the forward gears and of the reverse gear. The gearsets of forward gears 1 through 7 each have fixed gears and movablegears that can be shifted via hydraulically actuated gear selectors. Thegear selectors may be dual-synchronizer clutches, for example, each ofwhich is capable of switching two neighboring movable gears from aneutral position.

FIG. 2a shows the hydraulic system of the dual-clutch transmission in ahighly simplified block diagram. The hydraulic cylinders 22, 23 of theclutches K1, K2 and of the gear selectors are actuated by means of thehydraulic system. The hydraulic system of FIG. 2a has a high-pressurecircuit H and a low-pressure circuit N. In the high-pressure circuit H,the hydraulic cylinders 22, 23 of the clutches K1, K2 and of the gearselectors, connected therein, can be pressurized via a pressureaccumulator 25 with an accumulator pressure p_(S), which may be in therange of about 30 bar, for example. For this purpose, a main line 27connected to the pressure accumulator 25 leads along clutch paths 30, 31to the clutch hydraulic cylinders 23 and along gear selector paths 32 tothe gear selector hydraulic cylinders 22. Clutch valves or gear selectorvalves 35, 38 are positioned in each of the gear selector paths andclutch paths 30, 31, 32. The clutch valves or gear selector valves 35,38 are controllable in a manner not shown via a central control unit 39.In addition, the control unit 39 is in signal communication withpressure sensors 34. The pressure sensors 34 detect the hydraulicpressure applied to the first clutch K1 and to the second clutch K2.

The hydraulic system further comprises a charge pump 53, which isconnected on the input side to an oil sump 55. The charge pump 53 can beactivated by the control unit 39, via an electric motor 57, to chargethe pressure accumulator 25. In addition, the charge pump 53 is arrangedtogether with a cooling pump 59 on a common drive shaft 60, which isdriven by the electric motor 57. The cooling pump 59 is connected on itsoutput side to a low-pressure line 61, which leads to a distributionvalve 63. When a requirement for cooling exists, the hydraulic fluid canbe conducted to the first and/or to the second clutch K1, K2 andsubsequently back into the oil sump 55, dependent upon the position ofthe distribution valve 63.

In FIG. 2a , the main line 27 of the high-pressure circuit H branchesoff at a branching-off point 65 into a bypass line 67, which isconnected to the low-pressure line 61 of the low-pressure circuit N.Downstream of the branching-off point 65, a check valve 69 ispositioned, which will be described later. Also integrated into thebypass line 67 is an accumulator charging valve 71. The accumulatorcharging valve 71 can be adjusted between the charging position L shownin FIG. 2a and a cooling position K, depending upon the level of theaccumulator pressure p_(S) in the high-pressure circuit H.

The accumulator pressure p_(S) in the high-pressure circuit H acts as acontrol pressure, with which the accumulator charging valve 71 can beadjusted without additional external energy, i.e. automatically. Theaccumulator charging valve 71 is designed to move into the chargingposition L, for example when the accumulator pressure p_(S) in thehigh-pressure circuit H falls below a lower threshold value, for example25 bar. In addition, the accumulator charging valve 71 is automaticallyshifted into its cooling position K when the accumulator pressure p_(S)exceeds an upper threshold value p_(max), for example 28 bar.

During driving operation, actuations of the clutches K1, K2 and of thegear selectors G1 to G4 result in pressure losses. In addition, furtherpressure losses occur due to basic leakage in the high-pressure circuitH, i.e. due to leakage resulting from valve gaps or the like. As aresult, the accumulator pressure p_(S) is reduced during drivingoperation. If the accumulator pressure p_(S) should fall below the lowerthreshold value p_(min) (i.e., if a requirement to charge the pressureaccumulator exists), the accumulator charging valve 71 willautomatically move to its charging position L (FIG. 2). Upon detectionof the requirement to charge the pressure accumulator, the control unit39 will activate the electric motor 57 to a target charging speed. Thisenables the hydraulic charge pump 53 to charge the pressure accumulator25. In such a charging operation, the hydraulic charge pump 53 operatesunder a high pump load and therefore at a correspondingly high actualcurrent consumption I_(max) (FIG. 11). When the accumulator pressurep_(S) exceeds the upper threshold value p_(max) (FIG. 11), i.e. when arequirement to charge the pressure accumulator no longer exists, theaccumulator charging valve 71 automatically moves into its coolingposition K. In the cooling position K, the hydraulic charge pump 53delivers hydraulic oil via the now opened bypass line 67 into thelow-pressure circuit N. At the same time, the high-pressure circuit H isclosed in a pressure-tight manner via the check valve 69. Accordingly,the hydraulic charge pump 53 is no longer operating at a high pump load,but at a reduced pump load and also at a correspondingly reduced actualcurrent consumption I_(min) (FIG. 11).

As mentioned above, upon detection of a requirement to charge thepressure accumulator, the control unit 39 activates the electric motor57 to a target charging speed. For detecting such a requirement tocharge the pressure accumulator, a pressure sensor in the high pressurecircuit H and a position sensor in the accumulator charging valve 71 aredispensed with according to the invention. Instead, the control unit 39is equipped with an analysis unit. The analysis unit is in signalcommunication with a current measuring device 75, which is integratedinto control of the motor and which detects the actual currentconsumption I_(actual) of the electric motor 57, and with a speed sensor77, which detects the actual rotational speed n_(actual) of the electricmotor 57.

FIG. 2b illustrates the basic structure and the functioning of thepressure accumulator 25. According to said figure, the pressureaccumulator 25 is a piston-cylinder unit having an oil chamber 26, whichis connected to the hydraulic lines 27, 31, 32, and a preloaded pressurepiston 27. Preloading is achieved in this example by means of gaspressure applied to the pressure piston 27. Alternatively, preloadingmay be achieved by means of a spring. When the oil chamber 26 iscompletely drained, the pressure piston 27 (indicated in FIG. 2b by adashed line) is pushed with a preload force F_(V) against a stop 29 ofthe pressure accumulator 25. This means that during a filling operation,a hydraulic pressure which is greater than a preload pressure p_(V) thatcorrelates to the preload force F_(V) prevails, to overcome the preloadforce F_(V).

In FIG. 2b , the pressure accumulator 25 is shown in a partially filledstate, in which the hydraulic oil is acting on the pressure piston 27with an accumulator pressure, thereby building up the preload forceF_(V). In the completely drained state, the hydraulic lines 27, 31 arenot pressurized by the pressure accumulator 25. Rather, ambient pressureP_(U) prevails in the hydraulic lines 27, 31, 32. The automatictransmission is ready for operation when all the hydraulic lines 27, 31,32 are filled with hydraulic oil and when the hydraulic pressure in thehydraulic lines 27, 31, 32 is greater than the preload pressure p_(V),specifically by a preset pressure difference, so that the state ofoperational readiness will not be lost again due to basic leakage assoon as the charge pump 53 is switched off.

In FIG. 2a , the control unit 39 includes a diagnostic module 79 withwhich the charging behavior can be checked, more particularly,conditions can be checked to determine whether the actual preloadpressure p_(V) in the pressure accumulator 21 matches a referencepreload pressure pV_(Ref) indicated in the specification (i.e.structurally specified). The program blocks required for this areoutlined in FIG. 3. According to said figure, the diagnostic module 79has an analysis unit 80, which compares a temperature-dependent preloadpressure pV_(Ref) stored in a characteristic map 83 with an actualaccumulator pressure p_(S)(t_(V)) (FIG. 4), which will be describedlater. The actual accumulator pressure p_(S)(t_(V)) is detected by thepressure sensor 34 at a preload pressure time t_(V), which will bedescribed later. During the diagnostic operation, the clutch valve 35 inone of the clutch paths 30, 31 is constantly open, while the clutchvalve 35 in the other clutch path is closed.

If the pressure accumulator is functioning properly, the actualaccumulator pressure p_(S)(t_(V)) detected at the preload pressure timet_(V) will match the reference preload pressure p_(VRef). In contrast,if a significant preload pressure deviation exists, the analysis unit 80will identify this as a preload pressure fault, which will be stored ina preload pressure fault memory 81 (FIG. 3). If it is determined thatthe pressure accumulator 25 is functioning properly, a further analysisunit 82 (FIG. 4) of the diagnostic module 79 will perform a clutch pathdiagnosis, which will be described later.

In the following, the pressure accumulator diagnosis (i.e., preloadpressure diagnosis) and the clutch path diagnosis will be described inreference to FIGS. 3 and 4: To prepare for pressure accumulatordiagnosis, the oil chamber 26 of the pressure accumulator 25 is drainedcompletely and the actual accumulator pressure p_(S)(t) in the hydraulicsystem is reduced to an ambient pressure p_(U) so that the pressureaccumulator diagnosis can begin at a diagnosis start time t_(S) (FIG.4). The above-described condition for the start of diagnosis is achievedby actuating the hydraulic cylinders 22, 23 of the clutches K1, K2 andthe gear selectors G1 to G4, as indicated in the graph illustratingtravel distance at the top of FIG. 4. Accordingly, the hydrauliccylinders 22, 23 are activated intermittently by supplying power to therespective clutch valves or gear selector valves 35, 38 until, due tothe removal of hydraulic fluid associated with the hydraulic cylinderactuation, the accumulator pressure p_(S) detected by the pressuresensor 34 is reduced to the ambient pressure p_(U). The existence ofsuch an ambient pressure p_(U) can be detected by the pressure sensor34. Alternatively, position sensors 93 in the hydraulic cylinders 22, 23may be used to determine whether or not the respective hydrauliccylinder 22, 23 is still traveling a travel distance s (FIG. 4). If not,it will be concluded that an ambient pressure p_(U) exists in thehydraulic system.

Diagnostic charging operation, in which the hydraulic charge pump 53 isoperated at a constant charging speed n_(L) (FIG. 4, lower graph), thenbegins at time t_(S) (FIG. 4). First, for example, the pressure sensor34 located in the first clutch path 31 detects the actual accumulatorpressure profile p_(S)(t), as represented in the middle graph in FIG. 4.As illustrated by said graph, the accumulator pressure p_(S) increasesuntil the preload pressure time t_(V) at which the actual accumulatorpressure p_(S)(t_(v)) detected by the pressure sensor 34 has reached thepressure accumulator preload pressure p_(V).

As was stated above, if the pressure accumulator is functioningproperly, the actual accumulator pressure p_(S)(t_(V)) detected at thepreload pressure time t_(V) (accounting for temperature dependencies)will be identical to a reference preload pressure p_(VRef). If theactual accumulator pressure p_(S)(t_(V)) detected at the preloadpressure time t_(V) is found to deviate significantly from the referencepreload pressure p_(VRef), the analysis unit 80 will diagnose a preloadpressure fault. As diagnostic charging operation continues, after thepreload pressure time t_(V), the oil chamber 26 of the pressureaccumulator 25 is filled, specifically by displacement of the pressurepiston 27.

As is clear from the middle graph of FIG. 4, during diagnostic chargingoperation the actual accumulator pressure profile p_(S)(t) rises with asteep pressure gradient pi until the preload pressure p_(V) is reachedin the pressure accumulator 25 (i.e. up to the preload pressure timet_(V)). Afterward (i.e. after the preload pressure time t_(V)), incontrast, the actual accumulator pressure profile p_(S)(t) rises withonly a shallower pressure gradient {dot over (p)}₂. This characteristiccharging curve for the pressure accumulator 25 is used as follows todetermine the preload pressure time t₂: The analysis unit 80 detects thepressure gradients {dot over (p)}₁, {dot over (p)}₂ of the actualaccumulator pressure profile p_(S)(t). When a significant gradientchange between the pressure gradients {dot over (p)}₁ and {dot over(p)}₂ is detected, the analysis unit 80 identifies this as the preloadpressure time t_(V).

If no preload pressure fault is detected in the above preload pressurediagnosis, this will be followed immediately by the clutch pathdiagnosis: For this purpose, the diagnostic charging operation carriedout during the pressure accumulator diagnosis is simply continued untilthe pressure sensor 34 reaches an upper threshold value p_(max) (FIG. 4,middle graph). In the middle graph of FIG. 4, the upper threshold valuep_(max) lies above the preload pressure p_(V) of the pressureaccumulator 25 by a pressure difference Δp. When the diagnostic chargingoperation is completed, a second analysis unit 82 compares a pressuregradient {dot over (p)}₃ of the actual accumulator pressure profilep_(S)(t) with a reference pressure gradient {dot over (p)}_(Ref), whichis stored on a temperature-dependent basis in a characteristic map 84(FIG. 3) in the diagnostic module 79. Based upon this comparison, theanalysis unit 82 determines whether a fault-free or a faultyleakage-induced pressure decrease is present in the actual accumulatorpressure profile p_(S)(t).

It should be emphasized that the clutch path diagnosis is performed onlyif the analysis unit 80 does not detect a preload pressure fault. If thepressure accumulator 25 is fault-free, any faulty leakages can beunambiguously assigned to the clutch path 31. Both during the pressureaccumulator diagnosis and during the clutch path diagnosis, the pressurecontrol valve 36 located in the connecting line 37, which connects themain line 27 to the gear selector paths 32, is closed.

To validate the results obtained in the preload pressure/clutch pathdiagnosis, the diagnostic operation described above in reference to thefirst clutch path 31 can be performed twice, specifically as part of afirst partial diagnosis A using the pressure sensor 34 located in thefirst clutch path 31 and with the clutch valve 35 in the second clutchpath 32 closed. The above diagnostic operation can then be performed aspart of a second partial diagnosis B, specifically with the pressuresensor 34 located in the second clutch path 30 and with the clutch valve35 in the first clutch path 31 closed.

If the same fault is detected in both the first partial diagnosis A andthe second partial diagnosis B, the diagnostic module 79 can diagnose apressure accumulator fault and can rule out a clutch path fault withhigh probability. If different fault results are obtained, thediagnostic module 79 can diagnose a leakage fault in one of the twoclutch paths 30, 31.

FIG. 5 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for a gear selector pathdiagnosis. The gear selector path diagnosis is performed as a follow-ondiagnosis immediately following the clutch path diagnosis (FIG. 3) underthe condition that at least one clutch path 30, 31 is diagnosed ashaving fault-free leakage during the clutch path diagnosis. The pressuresensor 34 of the clutch path 30, 31 that is classified as fault-free(hereinafter referred to as the reference clutch path) is used for thegear selector path diagnosis illustrated in FIGS. 5 and 6.

As is clear from FIG. 5, the diagnostic module 79 has a third analysisunit 85, at the signal input of which an actual accumulator pressurep_(S)(t) detected by the pressure sensor 34 and an actual accumulatorpressure gradient {dot over (p)} are applied. The analysis unit 85checks the leakage behavior of each of the gear selector paths 32separately. Any leakage faults that are detected are stored in the faultmemory 87.

In the following, the gear selector path diagnosis will be described inreference to FIGS. 5 and 6: The diagnostic module 79 begins by openingthe clutch valve 35 located in the reference clutch path 30, in order todetect the actual accumulator pressure profile p_(S)(t). The pressurecontrol valve 36 in the connecting line 37 of the hydraulic system isalso opened, to establish a pressure connection between the pressuresensor 34 located in the reference clutch path 30 and the gear selectorpaths 32. A diagnostic charging operation is then performed byactivating the hydraulic charge pump 53. During the diagnostic chargingoperation, the actual accumulator pressure p_(S)(t) is increased up tothe upper threshold value p_(max) (FIG. 6) at the end time t_(off). Whenthe diagnostic charging operation has ended, i.e. at the end timet_(off) (FIG. 6), the pressure sensor 34 detects a pressure gradient{dot over (p)}_(K+G) of the accumulator pressure profile p_(S)(t) duringa measurement time interval Δt_(M). The analysis unit 85 compares thepressure gradient {dot over (p)}_(K+G) with a reference pressuregradient p_(Ref) and analyzes whether a fault-free or a faulty pressuredecrease (i.e. a gear selector leak) is present in the accumulatorpressure profile p_(S)(t).

As shown in FIG. 2a , each of the gear selector valves 35 located in thegear selector paths 32 can be adjusted between a closed valve position Sand two flow-through valve positions D1, D2. The gear selector pathdiagnosis is performed for each of the flow-through valve positions D1and D2 separately in the gear selector path 32 to be tested. This meansthat in each gear selector path 32, the gear selector diagnosis iscarried out both with the gear selector valve 38 in the firstflow-through valve position D1 and with the gear selector valve 38 inthe second flow-through valve position D2. In contrast, the gearselector valves 38 in the remaining gear selector paths 32 remainswitched to the closed valve position S, in order to increase measuringaccuracy in the diagnosis of the gear selector path 32 being tested. Thepressure gradient {dot over (p)}_(K+G) detected in the measurement timeinterval Δt_(M) by the pressure sensor 34 therefore reflects thecollective pressure decrease both in the reference clutch path 30 and inthe gear selector path 32 being tested, the gear selector valve 38 ofwhich is switched to one of the two flow-through positions D1, D2.

The reference pressure gradient p_(Ref) is read out from acharacteristic map database, e.g. from the characteristic map database83 already shown in FIG. 3. In this case, the readable referencepressure gradient p_(Re)r would correspond to a fault-free basic leakageof the reference clutch path 30. In the analysis unit 85, in addition tothe detection of the pressure gradients p_(K+G), absolute pressurevalues are also detected, i.e. the actual accumulator pressurep_(S)(t_(Start)) at the start time t_(Start) and the actual accumulatorpressure p_(S)(t_(End)) at the measurement end time t_(End) of themeasurement time interval Δt_(M). In this case, if the conditions aremet that, first, there is a sufficiently large accumulator pressuredifference between the start time t_(Start) and the end time t_(End),and second, the pressure gradient {dot over (p)}_(K+G) is equal to thereference pressure gradient {dot over (p)}_(Ref), the analysis unit 85will diagnose a fault-free gear selector path 32.

FIG. 7 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for an accumulator volumediagnosis. The accumulator volume diagnosis is performed as a follow-ondiagnosis immediately following the gear selector diagnosis (FIGS. 5 and6), under the condition that in the gear selector diagnosis, at leastone gear selector path 32 of gear selectors G1 to G4 has been diagnosedas fault-free and can thus be used as a reference gear selector path inthe accumulator volume diagnosis.

As is clear from FIG. 7, the diagnostic module 79 has an analysis unit89 which, in a comparator block 97, compares a hydraulic fluid removalY_(E) determined during the accumulator volume diagnosis with areference accumulator volume V_(ref). If a significant deviation isfound, an accumulator volume fault is diagnosed, which is stored in thefault memory 91. The reference accumulator volume V_(ref) can be readout from an accumulator volume characteristic map in a database, inwhich the reference values are stored on a temperature-dependent basis.

As is further clear from FIG. 7, the analysis unit 89 is in signalcommunication with a position sensor 93 of the gear selector hydrauliccylinder 22 located in the reference gear selector path 32. During theaccumulator volume diagnosis, the gear selector valve 38 in thereference gear selector path 32 is actuated, and the position sensor 93detects the travel distances Δs of the gear selector hydraulic cylinder22. These are integrated in a travel distance integrator 95 to obtain atotal travel distance s_(total). In a converter block 96, the totaltravel distance s_(total) is converted to a total displacement volumeV_(S). To the total displacement volume V_(S), a hydraulic fluid leakagevolume V_(L) that flows out during the accumulator volume diagnosis isadded. The resulting hydraulic fluid removal V_(E) is forwarded to theaforementioned comparator block 97.

The accumulator volume diagnosis is performed as follows: First, thepressure accumulator 25 is filled completely with hydraulic fluid in adiagnostic charging operation. The diagnosis charging operation is acharging process which takes place in a defined time t. The referencehydraulic cylinder 22 is then actuated intermittently within a diagnosistime interval Δt_(D), beginning at a start time t_(Start) (which in FIG.8 coincides with the switch-off time t_(off)) and continuing until, as aresult of the leackage volume V_(L) and the displacement volume V_(S)removed from the hydraulic system, an ambient pressure p_(U) is presentin the hydraulic system. Rather than being measured via a pressuresensor, the ambient pressure p_(U) is detected indirectly in thediagnostic module 79, specifically at the end time t_(end) (FIG. 8) ofthe diagnosis time interval Δt_(D), when, despite the flow-through valveposition D1, D2 of the reference control valve 35, the position sensor93 no longer detects any further travel distance Δs.

During the pressure accumulator volume diagnosis, one of the clutchpaths 30, 31 as the reference clutch path, along with the reference gearselector path 32 that leads to the reference hydraulic cylinder 22, ispressurized with the accumulator pressure p_(S) prevailing in thehydraulic system. In contrast, the hydraulic cylinders 22 of the othergear selector paths 32 and of the other clutch path are decoupled fromthe accumulator pressure p_(S).

The leakage volume V_(L) can be determined based upon the pressuregradients in the clutch path 30 and at the reference gear selector 22,detected during the preceding diagnoses (e.g. the pressure gradient {dotover (p)}_(K+G) from the gear selector path diagnosis according to FIGS.5 and 6). The pressure gradient {dot over (p)}_(L) is multiplied in theanalysis unit 89 by the diagnosis time interval Δt_(D). The resultingpressure difference Δ_(pL) is converted in a converter 98 to the leakagevolume V_(L).

FIG. 9 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for a switchover pointdiagnosis of the accumulator charging valve 71. The switchover timingdiagnosis is performed as a follow-on diagnosis immediately followingthe accumulator volume diagnosis (FIGS. 7 and 8), under the conditionthat a plausible accumulator volume of the pressure accumulator 25 wasdiagnosed in the accumulator volume diagnosis.

As is clear from FIG. 9, the diagnostic module 79 has an analysis unit105 with which, as part of the switchover timing diagnosis, a check ismade to determine whether a first switchover time t_(U1), at which theaccumulator charging valve 71 switches automatically from its chargingposition L to its non-charging position K, and a second switchover timet_(U2), at which the accumulator charging valve 71 switchesautomatically from its non-charging position K to its charging position,are plausible. For this purpose, the analysis unit 105 determineswhether, at the first switchover time t_(U1), the actual accumulatorpressure p_(S)(t) is within range of the upper pressure threshold valuep_(max). In addition, the analysis unit 105 determines whether at thesecond switchover time t_(U2), the actual accumulator pressure p_(S)(t)is within range of the lower pressure threshold value p_(min).

For detecting the two switchover times t_(U1) and t_(U2), the currentmeasuring device 75 of the electric motor 57 is used. The currentmeasuring device 75 detects the actual current consumption I(t) of theelectric motor 57. In this process, the time of a change from a highcurrent consumption I_(max) to a low current consumption I_(min) isdefined by the control unit 39 as the first switchover time t_(U1). Thetime of a change from the low current consumption I_(min) to the highcurrent consumption I_(max) is defined as the second switchover timet_(U2).

The clutch path pressure sensor 34 is used to detect the actualaccumulator pressure p_(S)(t). In FIG. 11, the measuring range Δp_(meas)of said sensor (FIG. 11) lies outside of, i.e. below the pressurethreshold values p_(max) and p_(min). Thus, a detection of the actualaccumulator pressure p_(S) immediately at the two switchover timest_(U1) and t_(U2) is not possible because the actual accumulatorpressure at the two switchover times lies outside of the measuring rangeΔp_(meas).

In FIG. 9, the actual accumulator pressure p_(S)(t) is determined at theswitchover times t_(U1) and t_(U2) by estimation, specifically with theaid of an extrapolation block 107. In the extrapolation block 107, basedupon measured pressure values p_(a)(t_(a)) and p_(b)(t_(b)) in theaccumulator pressure profile that are within the pressure sensormeasuring range (Δp_(meas)), a time frame Δt_(target) is estimated. Ifthe meas, accumulator charging valve is operating properly, the firstswitchover time t_(U1) will lie within the time frame Δt_(target). InFIGS. 9 and 11, the time frame Δt_(target) is bounded by the two timest₁ and t₂. In the subsequent comparator block 108 it is determinedwhether the first switchover time t_(U1) lies within or outside of thetime frame Δt_(target). If the first switchover time t_(U1) lies outsideof the time frame Δt_(target), a fault will be diagnosed, which will bestored in the fault memory 109.

FIG. 9 shows only a partial diagnosis in the program blocks, in which acheck is made to determine whether or not the first switchover timet_(U1) lies within the time frame Δt_(target). In the same manner, theanalysis unit 105 checks to determine whether or not the secondswitchover time tut lies within an estimated time frame.

FIG. 10 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for a valve spreaddiagnosis. The valve spread diagnosis is performed as a follow-ondiagnosis immediately following the switchover timing diagnosis (FIG. 9)under the condition that at least one plausible switchover time t_(U1)of the charging accumulator valve 71 has been identified in theswitchover timing diagnosis.

In FIG. 10, the diagnostic module 79 has an analysis unit 99 which, in avalve spread diagnosis, determines the actual valve spread Δp_(actual)between the lower pressure threshold value p_(min) and the upperpressure threshold value p_(max). A comparator block 101 of the analysisunit 99 compares the actual valve spread Δp_(actual) with a target valvespread Δp_(Ref). If a significant deviation is found, a fault isdiagnosed and is stored in the fault memory 103.

To determine the actual valve spread Δp_(actual), the analysis unit 99defines a diagnosis time interval Δt_(D). The diagnosis time intervalΔt_(D) begins at the first switchover time t_(U1) and ends at thesubsequent second switchover time t_(U2). Within the above-defineddiagnosis time interval Δt_(D), the diagnostic module 79 activates areference hydraulic cylinder 22, which according to FIG. 11 is switchedback and forth constantly, i.e. intermittently, during the diagnosistime interval Δt_(D). Due to the actuation of the reference hydrauliccylinder 22 and due to a system-inherent hydraulic system leakage, anaccumulator pressure decrease Δp_(E) that corresponds to the actualvalve spread Δp_(actual) occurs during the diagnosis time intervalΔt_(D).

The accumulator pressure decrease Δp_(E), i.e. the actual valve spreadΔp_(actual), is determined using the program blocks shown in FIG. 10, asfollows: From the position sensor 93, the piston travel distances Δs areintegrated in an integrator 94 during the diagnosis time interval Δt_(D)to obtain a total travel distance s_(total). This is then used in aconverter block 95 to calculate the pressure decrease Δp_(B) associatedwith the gear selector actuation. The pressure decrease Δp_(B)associated with the gear selector actuation is added in a summationelement to the leakage-induced pressure decrease Δp_(L), which gives theaccumulator pressure decrease Δp_(E) during the diagnosis time intervalΔt_(D). The leakage-induced pressure decrease Δp_(L) in the referencehydraulic cylinder 22 has already been determined in previous diagnoses.

As shown in FIG. 2a , connected upstream of the two clutch paths 30, 31is a safety valve 28 that can be activated by the electronic controlunit 39. The safety valve 28 can be actuated between a closed positionand a flow-through position. In the closed position, the two clutchpaths 30, 31 are pressure-decoupled from the pressure accumulator 25. Inthe flow-through position, the two clutch paths 30, 31 can bepressurized with the accumulator pressure p_(S). If the control unit 39detects a malfunctioning of the clutch valve 35 in at least one of theclutch paths 30, 31, the safety valve 28 can be adjusted to its closedposition for safety reasons. During normal driving operation, the safetyvalve 28 is constantly in its flow-through position.

FIG. 12 is a simplified block diagram showing the program blocks of thediagnostic module 79 that are required for a safety valve diagnosis. Thesafety valve diagnosis can be performed independently of otherdiagnostic steps. During the safety valve diagnosis, the safety valve 28is switched from the flow-through position to the closed position at adiagnosis start time t_(Start) (FIG. 13), thereby creating an actualpressure decrease Δp_(actual) downstream of the safety valve 28. Thediagnostic module 79 has an analysis unit 111 which compares this actualpressure decrease Δp_(actual) with a target pressure decreaseΔp_(target). If a significant deviation is found, a fault is diagnosedand stored in a safety fault memory 113.

The aforementioned clutch pressure sensor 34 can be used to detect theactual pressure decrease Δp_(actual).

In the following, the performance of the safety valve diagnosis will bedescribed in reference to FIGS. 12 and 13: For proper measuringaccuracy, the hydraulic pump 53 is activated to a constant speedn_(test) to ensure that there is sufficient accumulator pressure p_(S)in the high-pressure circuit H, which according to FIG. 13 moves betweenthe upper pressure threshold value p_(max) and the lower pressurethreshold value p_(min). The clutch valve 35 of a reference clutch path30 or 31 is adjusted to its flow-through position prior to theaforementioned start time t_(Start) by a time difference Δt, so that thepressure sensor 34 between the clutch valve 35 and the clutch hydrauliccylinder 23 can detect the actual pressure decrease Δp_(actual). Duringthe time difference Δt, rather than reading out the hydraulic pressureactually present at the clutch hydraulic cylinder 22 to the analysisunit 111 (FIG. 12), the pressure sensor 34 reads out an upper limitingpressure of the measuring range Δp_(meas).

At the diagnosis start time t_(Start), the safety valve 28 is switchedfrom its flow-through position D to its closed position S. The resultingpressure decrease p_(actual) is detected by the pressure sensor 34 andis compared in the analysis unit 111 with the target pressure decrease.

FIG. 14 is a highly simplified block diagram showing the program blocksof the diagnostic module 79 that are required for the delivery volumeflow diagnosis. The delivery volume flow diagnosis is performed as afollow-on diagnosis immediately following the accumulator volumediagnosis (FIGS. 7 and 8), under the condition that a plausibleaccumulator volume of the pressure accumulator 25 is detected in theaccumulator volume diagnosis.

As is clear from FIG. 14, a gear selector hydraulic cylinder 22, whichis connected to the pressure sensor 25 via the gear selector 32, is usedfor the diagnosis. Upstream of the gear selector hydraulic cylinder 22is a gear selector valve 38, which can be actuated by the control unit39 to adjust the hydraulic pressure applied to the gear selectorhydraulic cylinder 22. The gear selector valve 38 can be adjustedbetween two flow-through positions D1, D2 in order to displace a piston33 in opposing piston strokes over the indicated travel distances s₁, s₂and at piston speeds {dot over (s)}₁, {dot over (s)}₂ in the hydraulicactuating cylinder 22. In FIG. 14, the piston 33 divides the hydrauliccylinder into a piston rod-side working chamber and a working chamberopposite the first, the two of which are connected via hydraulic controllines 41 to the gear selector valve 38. A gear selector G1, not shown,can be actuated by means of the piston rod 43 of the gear selectorhydraulic cylinder 22. With such a gear selector actuation, theelectronic control unit 39 controls the gear selector valve 38 in aknown manner into one of the flow-through positions D1, D2, in order tomove the piston rod. The piston stroke is associated with a hydraulicfluid removal V₁, V₂ (displacement volume) from the hydraulic system.Because the internal geometry of the gear selector hydraulic cylinder 22is known, the respective displacement volume V₁, V₂ is known. Alsoprovided is a position sensor 93, with which the piston speed {dot over(s)}₁, {dot over (s)}₂ in the respective piston stroke can be detected.

In the following, the delivery volume flow diagnosis will be describedin reference to FIGS. 14 and 15: First, the hydraulic pump 53 is firstdeactivated within a pressure reduction time interval Δt_(R) (FIG. 15),while at the same time, the gear selector valve 38 is actuatedintermittently by the electronic control unit 39, as shown in the middletravel distance graph of FIG. 15. During the pressure reduction timeinterval Δt_(R), the gear selector valve 38 is actuated so as to movethe gear selector hydraulic cylinder 22 back and forth until, as aresult of leakage-induced hydraulic fluid removal and as a result ofactuation-induced hydraulic fluid removal (i.e., displacements V₁, V₂),the accumulator pressure p_(S)(t) is reduced to the ambient pressurep_(U). In this state, the accumulator 25 has been completely drained.This is followed immediately by the start (t_(Start)) of a diagnosistime interval Δt_(D). During the diagnosis time interval Δt_(D) acharging operation of the hydraulic pump 53 is carried out, in which thepump is actuated at different test speeds n₁ and n₂. At the same time,the control valve 35 is adjusted intermittently between its flow-throughpositions D1, D2. This causes the piston 33 in the gear selectorhydraulic cylinder 22 to move back and forth in the gear actuatorhydraulic cylinder 22 in opposing piston strokes over piston traveldistances s₁, s₂ and at piston speeds {dot over (s)}₁, {dot over (s)}₂.

The position sensor 93 detects both the individual travel distances s₁,s₂ per piston stroke and the piston speeds {dot over (s)}₁, {dot over(s)}₂ per piston stroke. In addition, the number a (FIG. 14) of pistonstrokes during the diagnosis time interval Δt_(D) is detected. Thesedata are forwarded to the signal input of a converter block 115, inwhich an average piston speed {dot over (s)}_(average) is calculatedfrom the number a of detected piston strokes. From the average pistonspeed {dot over (s)}_(average), an actual delivery volume flowV_(actual) is calculated in the converter block 115. In an analysis unit113 which is connected in terms of signal communication downstream, theactual delivery volume flow V_(actual) is compared with a targetdelivery volume flow V_(target), factoring in the respective test speedn₁ and n₂ during the diagnosis time interval Δt_(D). If a significantdeviation is detected in the analysis unit 113, a fault is detected,which is stored in the fault memory 117.

As is clear from FIG. 16, all of the fault memories 81, 83, 87, 91, 103,109, 117 are in signal communication with an analysis unit 120, intowhich the fault signals generated in the fault memories can be read. Inthe analysis unit 120, an analysis matrix is stored, in which the faultsignals from the fault memories 81, 83, 87, 91, 103, 109, 117 aremerged.

For a comprehensive hydraulic system diagnosis, the analysis unit 120uses the analysis matrix to analyze all the fault signals incombination. Thus in the analysis unit 120, a comparison of faultsignals with acceptable, i.e. fault-free, functional diagnoses isfinally performed, thereby enabling a qualified appraisal of thecomponents installed in the hydraulic system. This appraisal is possiblewithout dismantling of the hydraulic system and without external testingequipment/measuring technology. By testing components in the installedstate (in the vehicle), a shortening of repair and maintenance times, areliable detection of defective components, a decrease in repairs thatmust be repeated, and a savings on analysis test bench capacities arepossible without the effort associated with dismantling.

The invention claimed is:
 1. A hydraulic system for an automatictransmission of a motor vehicle, comprising: a high-pressure circuit, inwhich a pressure accumulator, at least one clutch, and gear selectorsand a hydraulic pump, which are operated by an electronic control unitand by which an accumulator pressure in the high-pressure circuit isincreased are arranged, wherein a clutch valve that is controlled by thecontrol unit is arranged in a clutch path between the pressureaccumulator and a clutch hydraulic cylinder of the at least one clutch,using which clutch valve a hydraulic pressure applied to the clutchhydraulic cylinder is adjusted, and wherein a safety valve that isoperated by the control unit is arranged upstream of the clutch valve,which safety valve decouples the clutch path from the pressureaccumulator with respect to pressure in a closed position and appliesthe accumulator pressure to the clutch path in a flow-through position,wherein the control unit has a diagnostic module, using which a safetyvalve diagnosis is carried out, in which the safety valve is switched ata diagnosis start time from the flow-through position into the closedposition, namely with actual pressure decrease downstream of the safetyvalve, and an analysis unit is provided, which compares the actualpressure decrease to a target pressure decrease and, if a deviationbetween the actual pressure decrease and the target pressure decrease isbeyond a predetermined threshold, detects a fault which is read out in asafety valve fault memory.
 2. The hydraulic system according to claim 1,wherein for detecting the actual pressure decrease, a pressure sensor isassigned to the electronic control unit, using which sensor thehydraulic pressure applied to the clutch hydraulic cylinder is detected,and which is arranged between the clutch valve and the clutch hydrauliccylinder.
 3. The hydraulic system according to claim 2, wherein fordetecting the actual pressure decrease, the clutch valve is adjustedinto a flow-through position with a time delay before the start time, sothat the pressure sensor detects the actual pressure decrease.
 4. Thehydraulic system according to claim 2, wherein an actual accumulatorpressure in the high-pressure circuit is modulated between an upper anda lower pressure threshold value, and a measurement range is outside theactual accumulator pressure, and the actual pressure decrease detectableby the pressure sensor corresponds to a pressure sensor measurementrange down to an ambient pressure.
 5. The hydraulic system according toclaim 1, wherein the hydraulic pump is in a charging operation duringthe safety valve diagnosis, and runs at a constant speed, to ensure asufficiently high accumulator pressure in the high-pressure circuit andcompensate for possible leakages.
 6. The hydraulic system according toclaim 1, wherein a gear selector hydraulic cylinder is decoupled fromthe high-pressure circuit with respect to pressure during the safetyvalve diagnosis.